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Uranium trioxide (UO₃), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The toxic, teratogenic, and radioactive solid may be obtained by heating uranyl nitrate to 400 °C. The gas is produced when uranium burns in oxygen or air, and condenses at standard temperature and pressure.

Its most commonly encountered polymorph, γ-UO₃, is a yellow-orange powder.

Cameco Corporation, which operates at the world's largest uranium refinery at Blind River, Ontario, produces high-purity uranium trioxide.

Health and safety hazards

Like all hexavalent uranium compounds (also called uranium(VI) compounds) UO₃ is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, radioactive substance, which may cause shortness of breath, coughing, acute arterial lesions, and changes in the chromosomes of white blood cells and gonads leading to congenital malformations if inhaled.

• UO₃-Material Safety Data Sheet.

Karyotyping for exposure to teratogens and cancer risks

Exposure to teratogens may be measured by karyotype tests such as those most often provided for biopsy and amniocentesis. Soluble and most partially-soluble uranyl compounds affect gonadal chromosomes in proportion to the extent that they affect white blood cell chromosomes. Uranyl poisoning causes immune system disorders and may cause cancer.

Because of the several-year delay in observed increases of teratogenisis, information about prognosis is currently uncertain. Trend information is not favorable to the afflicted, but may level out or decrease. The probability of decrease is similar to the probability of superexponential growth.

Many have recommended that the use of incendiary uranium munitions be avoided.

Urine isotope ratio tests

Some have suggested that uranyl compound exposure may be reliably classified with isotope ratios. However, the different isotopes translocate at different rates. The more-soluble uranyl compounds accumulate in cellular nuclei, where they are unlikely to return to the bloodstream or urine. Chromosomal karyotyping is the correct diagnosis procedure.

Chemistry

In addition to the monomeric gas which may be produced by uranium combustion, solid UO₃ may be obtained by heating uranyl nitrate (UO₂(NO₃)₂·6H₂O) at 400 °C. This conversion is used in the reprocessing of nuclear fuel, which begins with the dissolution of the fuel rods in HNO₃.

Uranium trioxide is also an intermediary compound in the conversion of sodium diuranate yellowcake (Na₂U₂O₇·6H₂O) to uranium hexafluoride and is shipped between processing facilities in the form of a UO₃ gel. In the jargon of the uranium refining industry, the chemical solution containing the concentrated uranium trioxide is called "OK liquor". Upon heating, this material liberates ammonia, giving UO₃.

Combustion product of uranium

UO₃ gas vapor is produced by combustion of uranium metal in air (Cotton, 1991.) The burning temperature reaches 2800 K (Mouradian and Baker, 1963.) UO₃ gas will eventully condense under normal atmospheric conditions.

Uranium-nitrogen salts UN and UN₂, form above 700 deg. C. Professor Simon Cotton (1991) Lanthanides and Actinides (New York: Oxford University Press) also writes, on page 127: "Aerial oxidation of any uranium compound eventually results in the formation of a uranyl compound."

Uranium trioxide gas molecules are a known intermediate in the chemical transition reactions forming U₃O₈ crystals from combustion products including uranium oxides. According to Wilson (1961), Ackermann et al. (1960) show that U₃O₈ crystals result from the two step process:

/₃ U₃O₈(s) + /₆ O₂(g) → UO₃(g) at T₁;

UO₃(g) → /₃ U₃O₈ + /₆ O₂(g) at T₂;

where T₂ < T₁.

The uptake of oxygen by U₃O₈ is "not infrequently ignored" (Gmelin vol. U-C1, p. 98.)

Individual UO₃ molecules will not decompose below the burning temperature of uranium in air, because uranium monoxide requires additional energy to form, as does the release of O₂ by a single UO₃ molecule. (Hoekstra and Siegel 1958; Wanner and Forest (2004) p. 98.)

Structure

The geometry of the uranyl ion has been the subject of much debate. The close approach of two oxygen atoms to uranium, with each linear O-U-O bond from 1.7 to 1.9 Å, prevents the close approach of a third or more. Bond angles are 90 and 180 degrees. d-p and f-p bonding have been suggested to explain the short U-O bonds. (Cotton, 1991)

The only well characterized binary trioxide of any actinide is UO₃, of which several polymorphs are known. Solid UO₃ loses O₂ on heating to give green-colored U₃O₈: reports of the decomposition temperature in air vary from 200–500 °C. Heating at 700 °C under H₂ gives dark brown uranium dioxide (UO₂), which is used in MOX nuclear fuel rods.

The most frequently encountered polymorph is γ-UO3, whose x-ray structure has been solved from powder diffraction data. The compound crystallizes in the space group I4₁/amd with two uranium atoms in the asymmetric unit. Both are surrounded by somewhat distorted octahedra of oxygen atoms. One uranium atom has two closer and four more distant oxygen atoms whereas the other has four close and two more distant oxygen atoms as neighbors. Thus it is not incorrect to describe the structure as , that is uranyl uranate. (Engmann, 1963).

Infrared spectroscopy of molecular UO₃ isolated in an argon matrix indicates a T-shaped structure (point group C_2v) for the molecule. This is in contrast to the commonly encountered D_3h symmetry exhibited by most trioxides. One could think of the compound as "uranyl monoxide", O. From the force constants the authors deduct the U-O bond lengths to be between 1.76 and 1.79 angstroms. The authors did not attempt to deduce bond angles. (Gabelnick, Reedy, Chasanov, 1970)

In the gas phase

At elevated temperatures gaseous UO₃ and O₂ are in equilibrium with solid U₃O₈.

/₃ U₃O₈(s) + /₆ O₂(g) ⇄ UO₃(g)

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 and 1500 °C. The vapor pressure of UO₃ is low but appreciable, about 10 bar at 980 °C, rising to 10 bar at 1400 °C (in the presence of 1 bar oxygen). At these partial pressures, uranium trioxide is monomeric. (Ackermann, 1960)

Uranium oxide in ceramics

UO₃-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured Fiestaware is a well-known example of a product with a uranium-based glaze. UO₃-has also been used in formulations of enamel, uranium glass, and porcelain.

Prior to 1960, UO₃ was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a Geiger counter if a glaze or glass was made from UO₃.

Related anions and cations

Uranium oxide is amphoteric and reacts as acid and as a base, depending on the conditions.

As an acid:

UO₃ + H₂O → UO₄ + H

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO₄). Uranates tend to agglomerate, forming diuranate, U₂O₇

As a base:

UO₃ + H₂O → UO₂ + OH

Dissolving uranium oxide in a strong acid like sulfuric or nitric acid forms the double positive charged uranyl cation. The uranyl nitrate formed (UO₂(NO₃)₂ˑ6H₂O) is soluble in ethers, alcohols, ketones and esters; for example, tributylphosphate. This solubilty is used to separate uranium from other elements in nuclear reprocessing, which begins with the dissolution of nuclear fuel rods in nitric acid. The uranyl nitrate is then converted to uranium trioxide by heating.

From nitric acid one obtains uranyl nitrate, trans-UO₂(NO₃)₂·2H₂O, consisting of eight-coordinated uranium with two bidentate nitrato ligands and two water ligands as well as the familiar O=U=O core.

See also

• Gulf War syndrome

Accidental teratogens:

• Depleted uranium
• Agent Orange
• Thalidomide

References

Peer-reviewed

• R.J. Ackermann, et al., "Free Energies of Formation of Gaseous Uranium, Molybdenum, and Tungsten Trioxides," Journal of Physical Chemistry, vol. 64 (1960) pp. 350-355
• Wilson, W.B. (1961) "High-Pressure High-Temperature Investigation of the Uranium-Oxygen System," Journal Of Inorganic and Nuclear Chemistry, 19, 212-222.
• Roberts, L.E.J. and A.J. Walter (1961) "Equilibrium Pressures and Phase Relations in the Uranium Oxide System," Journal of Inorganic and Nuclear Chemistry, 22, 213-229.
• Mouradian and Baker (1963) "Burning Temperatures of Uranium and Zirconium in Air," Nuclear Science and Engineering, 15, 388-394
• R. Engmann, P.M. de Wolff (1963) "The Crystal Structure of γ-UO₃," Acta Cryst., 16, 993.
• Chapman, A.T. and Meadows, R.E. (1964) "Volatility of UO_2+/-x and Phase Relations in the System Uranium Oxygen," Journal of the American Ceramic Society, 47, 614-621.
• Drowart, J., A. Pattoret, and S. Smoes (1964) "Heat of sublimation of uranium and consistency of thermodynamic data for uranium compounds," Journal of Nuclear Materials, 12, 319-322.
• Hoekstra, H.R. and Siegel, S. (1970) "Uranium-Oxygen System at high pressure," Journal of Inorganic and Nuclear Chemistry, 32, 3237-3248.
• Morrow et al. (1972) "Inhalation studies of uranium trioxide" Health Physics 23, 273-280
• Ackermann, R.J. and A.T. Chang (1973) "Thermodynamic Characterization of U₃O_8-z Phase," Journal Of Chemical Thermodynamics, 5, 873-890.
• S.D. Gabelnick, G.T. Reedy, M.G. Chasanov (1973) "Infrared spectra of matrix-isolated uranium oxide species. II: Spectral interpretation and structure of UO₃," J. Chem. Phys., 59, 6397.
• Rauh, E.G. and R.J. Ackermann (1974) "First ionization potentials of some refractory oxide vapors," J. Chem. Phys., 60, 1396.
• Stuart et al. (1979) "Solubility and Hemolytic Activity of Uranium Trioxide." Environmental Research 18, 385-396
• Green, D.W. (1980) "Relationship between spectroscopic data and thermodynamic functions; application to uranium, plutonium, and thorium oxide vapor species," Journal of Nuclear Materials, 88, 51-63.
• Cort, B., et al. (1987) "Infrared Characterization of Uranium Oxide Powders Using a Metal Light Pipe," Applied Spectroscopy, 41, 493-495.
• Chazel, V., et al. (1998) "Effect of U₃O₈ specific surface area on in vitro dissolution, biokinetics, and dose coefficients," Radiation Protection Dosimetry, 79, 39-42.
• Ansoborlo, E. (1998) "Exposure implications for uranium aerosols formed at a new laser enrichment facility: application of the ICRP respiratory tract and systemic model," Radiation Protection Dosimetry, 79, 23-27. Abstract: "... urine assay could be useful, provided that measurements are made soon after a known acute intake."
• Stradling, G.N., et al. (2000) "Treatment for actinide-bearing industrial dusts and aerosols," Radiation Protection Dosimetry, 87, 41-50.
• Chazel, V., et al. (2000) "Variation of solubility, biokinetics and dose coefficient of industrial uranium oxides according to specific surface area," Radiation Protection Dosimetry, 88, 223-231.
• Nakajima, K.; Arai, Y. (2001) "Mass-spectrometric investigation of UO{sub 3}(g)", Journal of Nuclear Materials, 294, 250-255.
• Ansoborlo, E. (2002) "Determination of the physical and chemical properties, biokinetics, and dose coefficients of uranium compounds handled during nuclear fuel fabrication in France," Health Physics, 82, 279-289.
• Stradling, N. et al. (2003a) "Optimising monitoring regimens for inhaled uranium oxides," Radiation Protection Dosimetry, 105, 109-114.
• Stradling, N. at al. (2003b) "Anomalies between radiological and chemical limits for uranium after inhalation by workers and the public," Radiation Protection Dosimetry 105, 175-178.
• Khaskelis, A.I. (2004) "Uranium oxide weathering: spectroscopy and kinetics," Transactions of the American Nuclear Society, 91, 890-891. Abstract: "During nuclear fuel fabrication or reprocessing stages of a nuclear fuel cycle, it is possible for small particles of uranium oxides to escape into the environment. This paper reports measurements of rates of oxidation of uranium dioxide particles in controlled gas environments using in-situ phosphorescence spectroscopy. Comparison is made with reduction and reoxidation of uranium trioxide...." From Schueneman, R.A. et al. (2003) Journal of Nuclear Materials, 323, 8-17.
• Sutton, M. and S.R. Burastero (2004) "Uranium(VI) solubility and speciation in simulated elemental human biological fluids," Chemical Research in Toxicology, 17, 1468-1480.

United Nations invitation-only

• H. R. Hoekstra and S. Siegel (1958) "Recent Developments in the Chemistry of the Uranium-Oxygen System," in the Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy, (Geneva: UN) 7, 394-400.

Other

• H. Wanner and I. Forest, eds. (2004) Chemical Termodynamics of Uranium (Paris: OECD and French Nuclear Energy Agency)
• Gmelin Handbook of Inorganic Chemistry, 8th ed., English translation, vol. U-C1 (1977), page 98
• «Gmelin Handbuch der anorganischen Chemiek» 8th ed., vol. U-C2, pp. 118-120
• Gilchrist, R.L., J.A. Glissmyer, and J. Mishima (1979) "Characterization of Airborne Uranium from Test Firings of XM774 Ammunition," Technical report no. PNL-2944, Richland, WA: Battelle Pacific Northwest Laboratory, November 1979.
• URANIUM and CERAMICS by Edouard Bastarache
• Uranium and Insoluble Compounds from the Occupational Safety and Health Administration; note that UO₃ is moderately soluble (Morrow, 1972.)
• Chemical data from NIST

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